In the recovery of hydrocarbons, such as oil and gas, from natural hydrocarbon reservoirs, extensive use is made of wellbore treatment fluids such as drilling fluids, completion fluids, work over fluids, packer fluids, fracturing fluids, conformance or permeability control fluids and the like.
In many cases significant components of wellbore fluids are thickening agents, usually based on polymers or viscoelastic surfactants, which serve to control the viscosity of the fluids. Typical viscoelastic surfactants are N-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammonium chloride and potassium oleate, solutions of which form gels when mixed with corresponding activators such as sodium salicylate and potassium chloride.
Conventional surfactant molecules are characterized by having one long hydrocarbon chain per surfactant headgroup. In the viscoelastic gelled state these molecules aggregate into worm-like micelles. Gel breakdown occurs rapidly when the fluid contacts hydrocarbons which cause the micelles to change structure or disband.
In practical terms the surfactants act as reversible thickening agents so that, on placement in subterranean reservoir formations, the viscosity of a wellbore fluid containing such a surfactant varies significantly between water- or hydrocarbon-bearing zones of the formations. In this way the fluid is able preferentially to penetrate hydrocarbon-bearing zones.
N-erucyl-N,N-bis(2-hydroxyethyl)-N-methyl ammonium chloride is a waxy solid at room temperature that is difficult to dissolve in water. It is conventional to deliver it to the wellhead in the form of a concentrated solution in isopropyl alcohol. This is then mixed with water (and other additives) to form the treatment fluid.
Potassium oleate, on the other hand, is usually formed in situ by the on-line mixing oleic acid and potassium hydroxide solutions. However, the transportation and subsequent rig-site pumping of concentrated potassium (or sodium) hydroxide solution is potentially hazardous and the exothermic acid-base reaction may result in excessive heating.
The application of viscoelastic surfactants in both non-foamed and foamed fluids used for fracturing subterranean formations has been described in several patent applications and patents, e.g. EP-A-0835983, U.S. Pat. Nos. 5,258,137, 5,551,516, 5,964,295 and 5,979,557.
The use of viscoelastic surfactants for water shut off treatments and for selective acidizing is discussed in GB-A-2332224 and Chang F. F., Love T., Affeld C. J., Blevins J. B., Thomas R. L. and Fu D. K., “Case study of a novel acid diversion technique in carbonate reservoirs”, Society of Petroleum Engineers, 56529, (1999).
U.S. Pat. No. 5,981,446 discloses a method of producing a fracturing fluid by the addition of a dry mix blend to the mixed water being pumped into a well. However, the viscoelasticity of the fracturing fluid was generated by the dissolution and subsequent crosslinking of a water-soluble polysaccharide, and not by surfactant molecules. Also, ensuring adequate hydration of the polymer and providing sufficient energy to mix the blend into the mixed water is problematic for polymer-forming blends.
Definitions
The terms “carbo”, “carbyl”, “hydrocarbo” and “hydrocarbyl”, when used herein, pertain to compounds and/or groups which have only carbon and hydrogen atoms.
The term “saturated” when used herein, pertains to compounds and/or groups which do not have any carbon-carbon double bonds or carbon-carbon triple bonds.
The term “unsaturated” when used herein, pertains to compounds and/or groups which have at least one carbon-carbon double bond or carbon-carbon triple bond.
The term “aliphatic”, when used herein, pertains to compounds and/or groups which are linear or branched, but not cyclic (also known as “acyclic” or “open-chain” groups).
The term “cyclic”, when used herein, pertains to compounds and/or groups which have one ring, or two or more rings/(e.g., spiro, fused, bridged). Compounds with one ring may be referred to as “monocyclic” or “mononuclear” whereas compounds with two or more rings may be referred to as “polycyclic” or “polynuclear”.
The term “ring”, when used herein, pertains to a closed ring of from 3 to 10 covalently linked atoms, more preferably 3 to 8 covalently linked atoms.
The term “aromatic ring”, when used herein, pertains to a closed ring of from 3 to 10 covalently linked atoms, more preferably 5 to 8 covalently linked atoms, which ring is aromatic.
The term “heterocyclic ring”, when used herein, pertains to a closed ring of from 3 to 10 covalently linked atoms, more preferably 3 to 8 covalently linked atoms, wherein at least one of the ring atoms is a multivalent ring heteroatom, for example, nitrogen, phosphorus, silicon, oxygen, and sulfur, though more commonly nitrogen, oxygen, and sulfur.
The term “alicyclic”, when used herein, pertains to compounds and/or groups which have one ring, or two or more rings (e.g., spiro, fused, bridged), wherein said ring(s) are not aromatic.
The term “aromatic”, when used herein, pertains to compounds and/or groups which have one ring, or two or more rings (e.g., fused), wherein said ring(s) are aromatic.
By “powder” we mean a form of solid matter composed of discrete, solid units which may be e.g. particles, grains, pellets or tablets.
By “viscoelastic” we mean that the elastic (or storage) modulus G′ of the fluid is greater than the loss modulus G″ as measured using an oscillatory shear rheometer (such as a Bohlin CVO 50) at a frequency of 1 Hz and at 20° C. The measurement of these moduli is described in An Introduction to Rheology, by H. A. Barnes, J. F. Hutton, and K. Walters, Elsevier, Amsterdam (1997).
By “viscoelastic surfactant” we mean surfactant that, in solution, is capable of forming a viscoelastic fluid.
By “straight chain” we mean a chain of consecutively linked atoms, all of which or the majority of which are carbon atoms. Side chains may branch from the straight chain, but the number of atoms in the straight chain does not include the number of atoms in any such side chains.
By an “oligomeric” or “oligomer” surfactant we mean that the structure of the surfactant is based on from two to eight (and preferably two to five) linked surfactant monomer subunits, each monomer subunit having a polar head group (which may be a cationic, anionic or zwitterionic group) and a C10–C50 organic (i.e. aliphatic, alicyclic or aromatic) tail group bonded at a terminal carbon atom thereof to the head group. Preferably the C10–C50 organic tail group is a hydrocarbyl tail group. The monomer subunits are linked in the oligomer either head group-to-head group or tail group-to-tail group. When they are linked head group-to-head group, the oligomer has distinct tail groups corresponding to the tail groups of the monomer subunits and a super-head group formed from the plural head groups of the monomer subunits. When they are linked tail group-to-tail group, the oligomer has distinct head groups corresponding to the head groups of the monomer subunits and a super-tail group formed from the plural tail groups of the monomer subunits.
Although the oligomer is defined above in relation to a chemically-corresponding monomer subunit, in practice the oligomer surfactant may not necessarily be synthesised from that monomer. For example, a synthesis route may be adopted in which monomer subunits are first oligomerised and the head groups are then changed to those of the desired oligomer surfactant. That is the head groups of the monomer subunits used in practice to form the oligomer may be different from the head groups of the monomer subunits to which the oligomer chemically corresponds. In another example, if the tail groups of the monomers actually used to form the oligomer are unsaturated, the oligomerisation process may involve the partial or total hydrogenation of those groups, particularly if the tail groups are linked in the oligomer.
Furthermore the tail groups of the monomer units actually used to form the oligomer may be aliphatic, but if the monomer units are linked in the oligomer tail group-to-tail group, the links formed between the tail groups in the super-tail group may be aliphatic, alicyclic or aromatic.